This invention relates to methods of etching masked substrates and, in particular, to methods for wet etching masked substrates anisotropically to produce straight etch walls without undercutting.
Etching of masked substrates is an important step in the fabrication of integrated circuits and optical devices. Integrated circuits are typically fabricated by forming a protective silica layer over a silicon workpiece, forming a patterned polymer resist mask over the silica by photolithographic techniques and etching away unmasked portions of the silica to permit further processing, e.g. doping, of the underlying silicon. Optical devices such as planner waveguide devices are often fabricated using similar techniques.
One of the early techniques for etching the unmasked silica was wet etching with hydrofluoric acid. Unfortunately wet etching techniques were usually isotropic in that they etched at substantially the same rate in all directions. Thus, in addition to etching down through the silica toward the silicon substrate, they would also etch laterally underneath the mask, undercutting the mask and restricting its ability to precisely define the regions to be etched.
In part to alleviate this problem of undercutting, dry etching techniques such as plasma etching and reactive ion etching have been developed. Plasma etching is the selective removal of material by reactive chemical species generated within a plasma glow discharge ambient. It has advantages of ease of operation, cleanliness and ease of control. In reactive ion etching (RIE), plasma etching is enhanced by attracting ions from the plasma onto the workpiece. The main advantage of RIE is that it proceeds primarily in the direction of impinging ions, etching relatively straight walls as distinguished from the undercut walls typically produced by isotropic wet etching and isotropic plasma etching.
Reactive ion etching, however, also has disadvantages. One disadvantage is that is tends to be non-uniform, both on a macroscale from one end of the workpiece to another, and on a microscale in the vicinity of fine features. Moreover, as microcircuit sizes shrink, RIE can also produce undercutting. Collisions with neutral atoms in the plasma can drive ions against the side walls of a trench being etched. Moreover the potentials used in RIE to accelerate ions can damage thin layers. Accordingly there is a need for an improved method of anisotropic etching.
The present applicant has discovered that the application of an etch-rate reducing liquid that selectively wets the mask can permit anisotropic wet etching. In a preferred embodiment, the application of a hydrocarbon liquid film to a masked silica surface permits wet etching of straight silica walls without undercutting the mask. It is believed that the oil selectively wets the polymer mask, but not the silica surface. The oil will thus be selectively present at the point where the mask meets the silica. Since the HF etchant does not dissolve the oil, the etching produces straight walls instead of undercutting the mask.